Thermal shock and creep resistant porous mullite articles prepared from topaz and process for manufacture

ABSTRACT

Particles of an interconnected three-dimensional network of bar-like topaz crystals are used as a reactant (with added silicon dioxide, or a mixture with added silicon dioxide and hydrated aluminum fluoride or added silicon dioxide, hydrated aluminum fluoride and alumina) to form mullite whisker honeycomb articles suitable as catalyst supports.

This is a division of copending application Ser. No. 07/727,207, filedJul. 9, 1991, now U.S. Pat. No. 5,252,272, which is a continuation inpart of Ser. No. 07/386,186, filed Jul. 28, 1989, now abandoned, andSer. No. 07/567,995, filed Aug. 16, 1990, now U.S. Pat. No. 5,173,349.

BACKGROUND OF THE INVENTION

This invention relates to ceramic articles based on a three-dimensionalinterconnected mullite whisker crystal network and is specificallydirected to the use of topaz crystals as a reactant in an in situsynthesis of mullite whiskers to densify and strengthen mullite whiskerarticles produced without conventional sintering. In particular, theinvention relates to the reaction in situ of interconnected topazcrystals and silicon dioxide or topaz crystals, silicon dioxide andhydrated aluminum fluoride in shaped green bodies, especially extrudedhoneycombs.

Shaped ceramic articles which exhibit a high degree of thermal shock andcreep resistance have a number of commercially important applications,for example, as diesel particulate traps, hot gas filters, molten metalfilters, substrates for exhaust catalysts, catalytic substrates forozone conversion, catalytic substrates for precious metal combustors(Catcom) used to power gas turbines and in metal matrix composites.Several major problems, however, hinder such uses of ceramics. First,ceramics are susceptible to damage, such as cracking, caused by thermalshock and/or creep. Another problem results from the fact that ceramicstructures frequently are difficult to machine or to join, making itdifficult to manufacture ceramic articles having complex shapes. This isespecially true of thin-walled structures such as honeycombs.

Mullite is widely used in numerous ceramic applications and is acrystalline aluminum silicate of the approximate empirical composition3Al₂ O₃.2SiO₂. Mullite is characterized by a distinctive diffractionpattern, but mullite occurs in distinctly different crystalline shapes,the most common being prismatic cigar-shaped crystals, sometimesreferred to as "acicular" form. This form of mullite occurs as "clumps"and may be obtained, for example, by firing clays. Mullite can also besynthesized as smooth elongated single crystals (whiskers). Whiskershave a significantly higher aspect ratio than the prismatic crystals inconventional mullite materials. Mullite can also be synthesized ashighly elongate fibers. Generally, all forms of mullite have many of theknown valuable properties of alumina, such as a high melting point and,in addition, exhibit other valuable physical and chemical properties.However, when mullite is formed as whiskers, the unusual strengthassociated with single crystals is PG,5 obtained. Topaz (AL₂ SiO₄ F₂) isusually obtained as stocky, bar-like crystals which have a significantlylower aspect ratio than mullite whiskers. Topaz is characterized by aunique x-ray diffraction pattern and can be converted to various formsof mullite by reactions with silicon dioxide.

The art is replete with suggestions to use various whiskers, includingmullite whiskers, to reinforce ceramics. Generally, in such use thewhiskers are employed as discrete, nonagglomerated crystals which areformed into composites by conventional sintering technology. Thisinherently limits the content of mullite in the ceramic articles soproduced and introduces potentially fluxing materials. Thus mullitearticles based on composites including addition of discrete singlewhiskers cannot be used at temperatures as high as those that puremullite can survive, and production of such composites necessitateshandling fibrous material.

Various methods have been suggested in the prior art for the productionof mullite in whisker form. Generally, these involve solid-solidreactions at high temperatures with evolution of gaseous by-product.Formation of mullite whiskers from the reaction of anhydrous aluminumtrifluoride (or aluminum trifluoride and alumina) and silicon dioxidewith a topaz intermediate is described in U.S. Pat. Nos. 4,910,172,4,911,902, and 4,948,766, all to Talmy et al. The reactants pass througha topaz crystalline phase before mullite crystals are formed. Accordingto the teachings of the patents, the solid reactants must be anhydrousand an anhydrous silicon tetrafluoride atmosphere must be present toform the mullite whiskers. In U.S. Pat. No. 4,948,766 a porous shapedpreform containing the reactants and an organic binder is converted to ahighly porous felt, exemplified in examples as small discs, the greenbodies going through a topaz intermediate stage without isolation of thetopaz intermediate. The initial green bodies are highly porous as arethe felts.

Our copending application, U.S. Ser. No. 07/386,186, and now abandoned,is directed to an in situ chemical route for making porous mulliteceramic parts in near net shape from preformed precursors. The inventionpermits the formation of highly complex shapes such as thin-walledhoneycombs, and it entails heating coherent green bodies containing amixture of finely powdered hydrated aluminum fluoride and silicondioxide in a molar ratio of approximately 12:13, along with a fugitivebinder, while sweeping the volatile reaction products including silicontetrafluoride and water from the bodies. The reactants form mulliteaccording to the following equation:

    12AlF.sub.3.xH.sub.2 O+13SiO.sub.2 =2 (3Al.sub.2 O.sub.3.2SiO.sub.2)+9SiF.sub.4 +xH.sub.2 O                (1)

X is suitably about 3 and can be as high as 9. At temperatures of about600° C.-800° C. a topaz intermediate (Al₂ SiO₄ F₂) is formed and above890° C. this reaction results in the formation of mullite whiskers. Thetopaz intermediate is not recovered in this process.

Mullite whiskers products obtained by reaction (1) are highly porous,typically about 80% porous, and thus they are relatively weak. However,many of the important potential commercial applications for high mullitewhisker content shaped articles require higher strength while stillpossessing desirable micro and macro-structures.

Summary of the Invention

It has now been found that useful mullite whisker articles can beproduced by preparing interconnected topaz crystals from nearstoichiometric mixtures of hydrated aluminum fluoride and silicondioxide in the form of coherent green bodies by an in situ process andthen using the interconnected topaz thus recovered (or interconnectedtopaz crystals from another source) as a reactant with additional silicaor silica and aluminum fluoride in the form a shaped bodies to formdensified mullite whisker articles by in situ synthesis.

In one embodiment of the invention, topaz crystals prepared from a nearstoichiometric mixture of hydrated aluminum fluoride and silicon dioxideare mixed with and formed into coherent green bodies, such as aspaghetti-like extrudate, along with a fugitive binder, and the bodiesare heated to temperatures in the range of about 600°-800° C. to formbodies in essentially the same size and shape as the green bodies, butcomposed of interconnected bar-like topaz crystals. During the reaction,evolved volatiles, including water and silicon tetrafluoride, are sweptfrom the solids. Since the topaz crystals are not whiskers, they areonly weakly interconnected. Thus the topaz bodies are not relativelyfrangible and can be ground, for example, to a size finer than 100 mesh,or the extrudate can be mixed in a ball mill with other ingredientswhich will result in the breakdown of the extrudates into small granulesof interconnected topaz crystals.

In practice of one preferred embodiment of the invention, the topaz soformed is then made into another shaped green body, preferably ahoneycomb, along with additional hydrated aluminum fluoride and silicondioxide, these materials being used in an amount, relative to the topaz,such that a portion of the silicon dioxide reacts with the topaz to formmullite whiskers, and another portion of the dioxide reacts with thealuminum fluoride, to form additional mullite whiskers. This green bodyis then fired while sweeping volatiles by means of air or nitrogen toform an interconnected network of mullite whiskers and produce anarticle of greater density and strength than is obtained by Reaction(1). In an especially preferred embodiment, the fired body derived fromthese two sources of mullite whiskers is further densified by beinginfiltrated with an alumina-silica sol precursor to mullite. Thiscomposite is fired to convert the sol to additional mullite which willnot be, however, in whisker form.

In another preferred embodiment of the invention, mullite whiskerarticles of greater strength and density are prepared by mixingagglomerates of topaz crystals, prepared as described above, with aquantity of silicon dioxide stoichiometric to form mullite along with afugitive binder. The mixture is formed into green bodies which are firedat a time and temperature sufficient to form mullite, e.g. 890° C. orabove, whereby all of the mullite whiskers are formed by the reaction ofthe interconnected crystal topaz precursor with the added silicondioxide. This leads to less porous (denser) articles than are obtainedby Equation (1), as well as articles that are denser than those obtainedby reaction of agglomerates of topaz crystals with appreciable amountsof added aluminum fluoride and silica dioxide.

A preferred use of honeycomb whisker products is as the substrate for aprecious metal catalyst for use with Catcom applications. In suchapplications substrate material of thermal shock resistance is requiredbecause it must survive thermal stresses induced by significant thermalgradients created as a result, for example, of emergency shutdowns ofthe gas turbine. The shutdowns are anticipated to occur numerous timesduring the lifetime of the generator. A simplified way of expressingthermal stress in a solid cylindrical body can be given by the followingrelation:

    σ=Eα(T.sub.s -T.sub.c)/(2(1-v))

where σ is thermal stress, E is the elastic modulus, α is thermalexpansion coefficient, v is Poisson's ratio, T_(s) and T_(c) are thesurface and center temperatures of the cylinder, respectively.

DETAILED DESCRIPTION OF THE INVENTION

In putting the instant invention into practice, the topaz reactant ispreferably prepared by mixing aluminum fluoride hydrate, e.g., AlF₃.3H₂O, with amorphous or crystalline silicon dioxide, preferably usingproportions substantially stoichiometric to form topaz without freesilicon dioxide, e.g., from about 54 to 62 parts by weight aluminumtrifluoride (anhydrous basis) to 42 parts by weight silicon dioxidecorresponding to about 1 (one) mole of AlF₃ per mole SiO₂. The aluminumfluoride and silicon dioxide are in particulate form. The mixture isthoroughly mixed with a temporary binder material such as methylcellulose, the binder either being added dry followed by addition of aliquid vehicle, preferably water, or binder is added as an aqueoussolution or dispersion. Sufficient liquid vehicle is added to provide amix of extrudable consistency. Prior to extrusion, the mixture isthoroughly mixed and extruded to form green bodies in a form amenable todrying and firing. Sufficient binder is used to achieve a greenextrudate of adequate strength to facilitate handling. Excellent resultshave been obtained by extruding the mixture into thin, spaghetti-likestrands which can be of any appropriate diameter, e.g., 0.8 mms. to 6.4mms. These strands can be chopped into pellets before drying and firing,or the strands can be dried and fired. Extrudates of interconnectedtopaz crystals are then crushed into finer particles when milled.

It is preferable to dry the green bodies before firing in order tostrengthen the green bodies by the gelation of a binder such as methylcellulose so that the pieces can be handled with more ease during theconsequent firing step. Firing is preferably carried out by a furnacewhich has capabilities of operating at temperatures as high as 1500° C.with capabilities of driving out the fluorine containing gases evolvedduring the topaz and mullite forming reactions. Removing of fluorinecontaining gases is accomplished by peristaltic pump(s) which are placedoutside the furnace but are connected via refractory tubing to the exitgate of the furnace. The pumps, when operational, suck the gases frominside the furnace outside and send gases to a scrubbing unit to convertthe products to relatively non-hazardous solid products such as NaF andSiO₂. Peristaltic pumps are needed to protect the furnace furniture andheating elements by removing the fluorine containing gases, particularlyHF, from the reaction chamber by minimizing the residence time of thesegases inside the furnace. As a result of this pumping action, silicontetrafluoride is continuously removed from the reaction chamber, andalso faster than it normally would.

In the practice of the present invention, the reactants to producemullite including interconnected topaz crystals preferably formed asdescribed above, are mixed thoroughly with silicon dioxide or silicondioxide and hydrated aluminum fluoride along with a suitable binder,such as methyl cellulose and water. Other suitable binders include,e.g., alginate, polyethylene oxides, resins, starches, guar gum andwaxes. Choice of suitable binders for making the topaz intermediate aswell as the mullite whiskers article are discussed in U.S. Pat. No.4,551,295, the teachings of which are incorporated herein by reference.Following mixing with a binder the reactants are formed into a desiredshape using extrusion, injection molding, low pressure injectionmolding, pressing, tape casting or any other suitable ceramic processingtechnique.

Aluminum oxide may also be added to the reactive ingredient mixcomprising aluminum fluoride hydrate, silicon dioxide and topaz as fineparticles and at a weight ratio of 1 to 10% based on the total dryweight of the reactive ingredients, i.e., aluminum fluoride hydrate plussilicon dioxide, topaz and alumina. Alpha alumina is suitable, althoughother forms can be used. The main reason for aluminum oxide addition isto have it react with unreacted silicon dioxide at elevated temperaturesto form mullite particulates. A portion of the silicon dioxide mayremain unreacted because a portion of aluminum fluoride may volatilizebefore it reacts with silicon dioxide. In cases where stoichiometricamounts of aluminum fluoride and silicon dioxide are added to thereactive ingredient mix, this would result in excess silicon dioxide,which may not be desired in the final product. Excess aluminum oxidewill then react with this silica to form mullite particulates. Whenaluminum oxide is added to the reactive ingredient mix, some portion ofit would react with this excess silica while the remainder may stayunreacted. Thus the final fired honeycomb product may contain unreactedaluminum oxide.

In carrying out the invention, formed pieces of the powdered reactantsand binder are fired to a desired temperature (900° C. and above) in aflowing stream of air or nitrogen to sweep volatiles including but notlimited to silicon tetrafluoride and water, and, while the originalshape is retained, the precursor mix is transformed to mullite whiskers.There is no matrix, and the product is in the form of very porous shapedarticles consisting essentially of interconnecting and branched mullitewhiskers.

Inherent in this method of manufacturing mullite is the possibility thatthe fired articles may contain trace amounts of fluoride ions. Sinceeven very low concentrations of fluoride ion can poison many catalysts,it is important that the formed mullite articles be essentiallyfluoride-free, if they are to be used as support for metallic catalysts.One aspect of the present invention involves preparation of mullitearticles by the described route, and obtaining such articles in afluoride-free condition. Practical methods of fluoride ion removalinclude the use of steam or superheated steam. For example, a honeycombuseful as the support for metallic catalyst can, after it is fired, beimmersed in a super-heated (900° C.) concentrated sulfuric acid bathand/or have steam passed therethrough to remove fluoride ion.Alternatively, a hydrogen purge is an efficient remover of fluoride ion.The result is an essentially fluoride-ion-free mullite honeycombsupport.

The method of making porous mullite articles described here isespecially attractive because by this route shaped articles with veryhigh thermal shock resistance can be produced at comparatively low cost.The articles are lightweight and have very high creep resistance.According to the invention, articles with complex or thin shapes can beproduced with relative ease. Examples are honeycomb shapes, corrugatedsheets, reticulated (comb-shaped) pieces, foams, donuts or any otherdesired functional shape chosen to serve as a filter, catalyticsubstrate, particulate trap or other functional purpose. These articlescan be made into a near net-shape and/or machined extensively withoutcracking. Catalytic substrate walls with high roughness can be produced,and high specific strength (strength/density) mullite articles attainedwithout any residual glassy phase present.

Refractory catalyst supports such as those described in U.S. Pat. No.3,565,830 (incorporated herein by reference) constitute an advantageousutilization of the present invention. Mullite supports preparedaccording to the present invention may be readily coated withcatalytically active oxide, such as alumina, and then impregnated with aplatinum group metal for use with catalysts such as those described inthe reference patent.

Refractory catalyst supports are also required for process directed tocatalytically supported thermal combustion (Catcom) described forexample in U.S. Pat. Nos. 3,928,961 and 4,893,465 (incorporated hereinby reference). Mullite catalyst supports made according to the presentinvention are exceptionally well adapted for use in catalytic combustionprocesses such as those described in these patents.

Two articles made by the precursor mix according to the presentinvention can be joined to each other via thermal treating. Preparing agood joint between two similar materials has advantages. Certaincatalytic applications require large size of honeycomb pieces. A dieselparticulate filter can be about 12 inches in diameter or more. A largepiece of honeycomb shaped article is required for catalytic combustionapplications. Extrusion of such large pieces is very difficult. Sincethe joint between two articles made from this material has good quality,then large pieces can be assembled by joining smaller extruded parts.For example, a cylindrical honeycomb material can be assembled by fusingfour pieces of quadrant cylinders. In this method, several small piecesof green particulate aluminum fluoride, silicon dioxide and binderadmixture are joined either by placing in integral contact or pressingsmaller unfired pieces together before firing, so that upon firing largearticles of relatively more complex shapes are formed. See Examples 4and 16. Articles made with a topaz reactant, e.g. Example 16, to form ajoint are preferred because of the increased density.

Pure mullite ceramics of the present invention have a very high meltingpoint (>1880° C.). Production of articles composed of whiskers is anespecially straightforward and clean process, since there is no need todisperse mullite or other whiskers, the whiskers being formed duringheating of the shaped article made of the precursor mix, thus avoidinghealth and safety problems associated with respirable whiskers.

Clay can also be used in small amounts to improve the extrudability of ahoneycomb. Clay is more plastic than either silica or aluminum fluorideor interconnected topaz. The increased plasticity of the extrudate mixwhich accompanies clay addition enhances the ease of extrusion. Suchclay can be added to the dry mix or entrapped in the binder solution.Suitable clays include, for example, bentonite, attapulgite,palygorskite, montmorillonites, pyrophyllite and kaolin.

A further aspect of the invention involves improving the strength ofmullite articles by grinding the reactants prior to conversion to topazand prior to mixing the topaz with other reactants to form mullitewhiskers. Grinding of reactants to particle sizes of less than about 45microns can improve the compression strength of the mullite articles.Such grinding or particle size reduction can be conducted by a varietyof methods, ball milling being one of the more convenient well knownmethods. Ball mixing will also serve to reduce a spaghetti-likeextrudate of interconnected topaz crystals to small granules, e.g. finerthan 45 microns, of interconnected topaz.

Finally, as is appreciated by those skilled in the art, whiskerprocessing has the potential for creating serious health hazards. In themanufacturing route of the present invention, whiskers are formedin-situ from powders at elevated temperatures and remainedinterconnected. Thus, there is no need to handle loose whiskers duringthe manufacture route which is the subject of the instant invention.

Following is a detailed explanation of factors to be considered inselecting ingredients used in practice of the invention, especially assuch factors affect the micro-structure and macro-structure of mullitewhisker products of the invention.

The mullite whisker articles of our copending application Ser. No.(07/386,186) are made of mechanically interconnected mullite singlecrystal whiskers and have low elastic modulus. Low elastic modulus ismainly due to the very open nature of the product. An article made byreacting aluminum fluoride trihydrate and silica in accordance withEquation(1) is 80% or more porous. In a typical reaction: ##STR1## Atotal of 2436 grams of solids yield 852 grams of solid mullite. Thisnumber suggests that 65% of the starting solids react to form gaseousproducts. However, additional porosity is imparted by evolution ofvolatile byproducts derived from agents used in binding these reactantsto a new net shaped article. Thus the reactants are mixed with a binder,preferably an organic binder, such as methyl cellulose, in a weightratio preferably between 2-8 weight percent. A liquidbinder/plasticizer, preferably water, is added to the mixture at aweight ratio between 10-30%, and typically about 20% to facilitateforming. The mixture is then homogenized to a dough by using one ofseveral available mixing techniques, such as pug milling followed byextruding the mix through a die orifice to form honeycomb shapes. Thehoneycomb is then dried and fired in a furnace to complete the reactionto mullite.

The extruded green body obtained by reaction (1) is never 100% dense.There will be some porosity in the body, depending on the quality ofdispersion, particle size distribution, extrusion pressure, die shapeand some of the extruded parameters. Porosity of the green body willvary between 1-15%, which is significantly less than that of green discsof U.S. Pat. No. 4,984,766. If one assumes 5% porosity in the greenhoneycomb (i.e. the actual density is 5% less than the theoreticaldensity) and assumes water and binder contents of 20% and 5% of thereactant solids, and the density of the fired piece is expected to be23% of the theoretical density of mullite. As an example, 1656 grams ofaluminum fluoride trihydrate, 780 grams of silicon dioxide, 487 grams ofwater and 122 grams of binder are mixed and formed to a honeycomb, whichis then fired to obtain 852 grams of mullite. The product is only 28% ofthe original weight. Combined with the 5% inherent porosity, the mullitewhisker honeycomb is about 23% dense or about 77% porous. Thiscalculation suggests that fired honeycomb pieces of our copending patentapplication are 80% porous. Measurements confirm this estimate.

A very important factor involved in the preparation of mullite whiskerhoneycomb is that the fired honeycomb must retain the shape of the greenone despite the fact that the green body loses a significant portion ofits original weight. If the microstructure of mullite obtained from thereaction were such that the mullite grains were dense and equiaxed,i.e., more or less isotropic in three dimensions, then there could notbe strong connectivity between each grain. With lack of strong3-dimensional connectivity, the original honeycomb shape could not beretained. The product would turn into loose powder form upon firing, andcollapse from the honeycomb shape. A rigid honeycomb shape is obtainedupon firing because of the unique microstructure as shown in FIGS. 2-4.The mullite crystals are formed in the shape of whiskers. Each whiskeris dense, single crystalline which is branched as shown in FIG. 4, withthe whiskers being mechanically connected to another making a3-dimensional rigid body. Whiskers are anisotropic. Their aspect ratiois high, generally 100 or more. A three-dimensional body made ofinterconnected whiskers can have 80% or more void space and still berigid. This characteristic of the mullite made from the hydratedaluminum fluoride route is unique. Any dense, solid mullite powder,single or polycrystalline, but equiaxed with very little or noanisotropy, cannot produce a 3-dimensional rigid body with 80% voidspace.

The highly open nature of the microstructure leads to desirable lowelastic modulus. Porosity, in general, decreases elasticity of a ceramicmaterial. There is no well understood relation between the elasticmodulus and porosity, but several models have been developed and existin the literature. One model is described by:

    E=E.sub.o e.sup.-bP

    or

    E=E.sub.o exp(-bP)

where E_(o) is elastic modulus of the 100% dense body, P is porosity, bis an empirical constant. The relation suggests that E is decreasedsignificantly with porosity. Low E values are desirable in a Catcomsubstrate. Lower E means lower thermal shock susceptibility. A materialcomposed of dense solid single crystals, which can be formed in 80% ormore void space due to highly anisotropic nature of its individualgrains, and where these grains are interconnected to give rigidity tothe body is a good candidate for Catcom application due to itsexceptionally low elastic modulus.

However, strength of the substrate is another key parameter. Strongersubstrates have more chance to survive service during use. The mullitewhisker material obtained from reaction (1) is weak. Our copendingapplication teaches the substrate obtained by that reaction can bestrengthened by one of several routes or by their combination. However,none of these routes provides the multiple benefits obtained by growingmullite crystals from agglomerates of interconnected topaz.

In accordance with the instant invention, bar-like topaz crystals,preferably interconnected topaz crystals, are added to the mix. Theinterconnected topaz is preferably formed at a temperature between600°-800° C. via the following reaction: ##STR2## Topaz crystals formedby this reaction are stocky or bar-like and interconnected. At highertemperatures, topaz reacts with silica to form interconnected mullitewhiskers via reaction. ##STR3## Reaction 2 suggests that loss of weightis about 27%. This number is significantly lower than that obtained fromReaction 1. Thus if a substrate is made via Reaction 2, theinterconnected mullite whisker product is much denser as shown bycomparison of in the micrographs in FIGS. 9 and 10. The strength of themullite substrate obtained from topaz is therefore higher than thatobtained from aluminum fluoride trihydrate and silica.

As noted above, increasing density increases elastic modulus which thenincreases thermal shock susceptibility. Thus, there is a trade-offbetween density and strength. Rewriting the equation above for therelationship of factors involving thermal stress in solid body, it canbe calculated that optimization of the product for thermal shocksusceptibility is achieved by maximizing the σ/E ratio.

The densest mullite whisker product can be obtained by Reaction (2) andis hereinafter denoted T-100, meaning that the product is composed ofinterconnected whiskers produced by using interconnected topaz andsilica only. The product obtained by Reaction (1) is denoted by T-0,since no topaz was added to the original reaction mix. A T-25 productmeans appropriate amounts of topaz and silica dictated by Reaction (2)are added to a mixture of aluminum fluoride trihydrate and silicadictated by Reaction (1) such that, upon firing, 25% of theinterconnected mullite whiskers are obtained by the reaction of addedtopaz powder with silica (Reaction 2) while the rest of the whiskersoriginate from the reaction of hydrated aluminum fluoride with silica(Reaction 1).

Theoretical porosity contents for mullite whisker materials werecalculated using several assumptions namely:

a) The extruded green honeycomb is 95% dense, i.e. it retains 5%porosity.

b) The extruded green honeycomb has 5% organic binder by weight.

c) The extruded green honeycomb has a total 20% water and 5% liquidplasticizer by weight of solid reactants.

Based on these assumptions, theoretical porosity contents are:

    ______________________________________                                        MATERIAL      POROSITY, %                                                     ______________________________________                                        T-0           77                                                              T-25          73                                                              T-50          67                                                              T-75          59                                                              T-100         46                                                              ______________________________________                                    

These calculated porosity values clearly indicate that mullite whiskerhoneycombs become denser as their topaz content increases. This is dueto the fact that more fluoride is lost in AlF₃.3H₂ O than in Al₂ SiO₄ F₂and also that there are 3 moles of water in a hydrous aluminum fluoridemolecule whereas there is no water in anhydrous topaz. At hightemperatures water in fluoride will evaporate, causing significantmaterial loss from the reactive ingredient mix. Also fluoride will belost at even higher temperatures, causing additional loss from the solidphase. Thus, as the aluminum fluoride trihydrate content of the startingmixture decreases (i.e. T-number increases) there will be less loss ofmatter and the final product becomes denser. Within the scope of theinvention are products prepared from mixes in which topaz (T) content isdefined by the equation T-X, in which X is greater than 0. T-25 throughT-100 articles (about 50% to 75% porosity) are preferred and T-50through T-100 (about 45% to 70% porosity) are especially preferred.

Addition of topaz to the reactive mix is not the only way to increasedensity of the honeycomb. However topaz is unique because the densifiedbody is still in the form of a body composed substantially completely ofinterconnected whiskers.

Thus, another approach to densify and strengthen an interconnectedmullite whisker substrate is to increase its weight after firing. Thistechnique can be used to further densify mullite whisker productsobtained in accordance with this invention from a topaz reactant.Densification can be accomplished by different techniques. One isinfiltration. In this case, fired mullite whisker honeycombs areinfiltrated with a slurry or the like by dipping. The assembly is thendried and subsequently calcined. The composition of the slurry can beany form of aluminosilicate, or aluminum or silicon based compound,including clays. Preferred is the use of a mixture of alumina silicasols in proportion stoichiometric to form mullite. The slurry is thendried and calcined at a suitable sintering temperature, time andatmospheric condition. The calcined substrate is denser and presumablystronger than the uncoated substrate. A colloidal solution can also beused instead of a slurry. Chemical vapor deposition, chemical vaporinfiltration, sputter deposition or any other coating technology mayalso be suitable for densification. Another densification route is toblend the reactive ingredients with densification aids or fillers andthen extrude this mixture, dry and calcine the honeycomb. Theseadditives can be alumina, silica, zirconia or mullite powders, mullitewhiskers or any other silicon, aluminum or zirconium based compoundsincluding clays. Illustrative examples demonstrate producing mullitewhisker products made with additives such as clays, alumina, mullite orsilica sols as part of the reactant mixture and using mullite precursorsol as an infiltration vehicle on the fired honeycomb that can also beapplied to whisker products having a topaz precursor.

However, densification increases elastic modulus which is not desiredbecause higher elastic modulus increases thermal shock susceptibility.Thus, a trade-off between strength and thermal shock susceptibility mustbe made to produce honeycombs which have both adequate strength andthermal shock resistance.

Another key physical requirement for a substrate is high temperaturestrength and durability. Catcom substrate is operated at hightemperatures such as 1250° C. and above in steady-state conditions andfor long durations. It is also subjected to steady stress levels. Thepressure of the flowing gas can be as high as 10 atmospheres. Thepressure acts as stress on the face of the substrate. Thus, theconditions of creep, i.e. time dependent deformation of a material understress at elevated temperatures, exist in Catcom application. The choiceof material should address this issue. The material should possesssufficient strength at elevated temperature but must also be creepresistant.

Example 6 demonstrates that only a 10% drop in strength was observed at1300° C. for a T-0 article. The drop was limited to 25% of the roomtemperature strength when a sample of T-0 was tested at 1400° C. It isclear from this example that this mullite whisker material is capable ofwithstanding high temperatures in short term, and this is expected ofarticles ranging from T-0 through T-100. This is expected because themelting point of mullite is around 1800° C. and all of these articlescan be prepared so that they are composed entirely of mullite. Most ofthe commercial mullite products have limited use at temperatures inexcess of 1300° C. This is because of the viscous deformation of theglassy phase which is common to see along the grain boundaries ofmullite and is present because of sintering techniques generally used inpreparing mullite ceramics. This glassy grain boundary phase becomesless viscous at elevated temperatures when operated under a steadystress. The viscous relaxation which takes place microscopically createsa damaged zone at the high stress concentration regions causing grainboundary sliding, void growth, etc. These time dependent deformationphenomena called viscous creep, weakens the material and eventuallyleads it to failure even when it is operated at a low stress level. TheTEM microscopy done on the mullite whiskers showed that each individualwhisker is single crystalline and the region where two whiskers connectis free of glassy phases (FIGS. 4 & 5). Thus, the mullite whiskermaterial is substantially more creep resistant compared to most of theconventional mullite products.

Another form of long term failure is corrosion related. At hightemperatures the atomic mobility of species, or diffusivity, increasescausing corrosion or stress corrosion. These phenomena can limit the useof a refractory material which has otherwise good properties such ashigh strength, thermal shock and creep resistance. Washcoat degradationdue to diffusion of species from or to the substrate, or segregation ofcationic or anionic species present in the substrate composition towardsgrain boundaries are among the possible causes of degradation. Mullitematerial has an advantage here. Mullite is a mixed oxide material withvery high degree of covalent bonding for an oxide. Diffusivity of Al, Siand O are low. The mullite whisker material is pure and prepared with nocationic additives. Thus, diffusivity of Group I or II cations is not amajor problem. Mullite whisker material is resistant to corrosionrelated long term failures.

Gas turbines vary in size and capacity. Some systems require a substrateas wide as 22" in diameter. This size is very difficult to extrude. Oneway of producing large substrates is to make smaller ones and join them.Smaller mullite whisker substrates can be extruded and joined to formlarger pieces. Mullite whisker material can be joined relatively easily.Example 4 demonstrates that mullite whisker articles can be joined toform larger pieces and this can be achieved using topaz-derivedproducts. See Example 16. Topaz derived in situ joints are preferredbecause the mullite thus derived will be denser and the joint stronger.Whiskers from both of the pieces to-be-joined grow and mechanically jointo each other producing a clean joint boundary.

As the temperature difference between the surface and the center of thesubstrate becomes larger, the radial thermal stress induced on thesample increases. Substrate breaks when the induced stress reaches thestrength of the material.

The temperature differential, thus thermal stress, depends on the sizeof the catalytic unit. Obviously, units with larger diameters wouldexperience larger temperature differential and thermal stress. Theseverity of thermal shock varies from one gas turbine manufacturer toanother. However, regardless of the type and size of the powergenerator, the Catcom substrate should be designed to survive theemergency shutdown trips.

The equation for thermal stress given above shows that variables such asE, v and α are the key physical parameters in determining the inducedthermal stress at a given T. A candidate material should possess highstrength, low elastic modulus and thermal expansion coefficient so thatthe magnitude of thermal stress will be low and smaller than thestrength of the material to avoid failure during emergency shutdown.

In addition to the microstructure factors affecting design of substratesfor applications such as Catcom, there are a number of important designrequirements for the substrate which carries the catalyst in Catcom.These are macrostructure factors. The support should be manufactured inlarge sizes such as 22" in diameter; it should have uniform crosssection; it should possess high open frontal area such as 70% or more.

It is essential that momentum profile (mass×velocity) of the incominggas across the face of the turbine should be uniform for properfunctioning of the system. Uniform velocity profile is necessary tomaintain constant momentum. Honeycomb structures are used to smoothvelocity profiles of flow streams in many applications including gasturbines. Thus, a Catcom substrate made into a honeycomb structure hasan advantage over other shapes in improving the incoming gas stream andmaintaining its flow uniformity. Uniformity in the cell walls of thehoneycomb is also very critical. Gas will flow with higher velocitythrough a larger cell. The variation of velocity from one cell toanother should be avoided. Honeycombs can be manufactured by a number oftechniques. Extrusion process produces uniform cell sizes across theface of the substrate and is the preferred method of manufacturing. Anyother method used to make Catcom substrate must produce uniform cellsizes in order to be compatible with extrusion. Uniformity in the cellsize is essential not only for maintaining a uniform velocity profilebut also to maintain constant temperature across the outlet face of thecatalyst. It is essential for operating conditions that the gas phaseshould have a uniform temperature profile when it reaches the turbine.The quality of combustion is affected by cell size. The boundary layer,defined as the stagnant film of gas between the washcoat and the gasphase, is thicker in the larger cell. The honeycomb walls are the heatsources to the system. Heat transfer from the honeycomb walls to the gasphase becomes harder with the increased thickness of the boundary layer.Same is true for mass transfer. Thus, reaction is slower in the moreopen cells. If nonuniformity exists in the honeycomb, unequal reactionrates will occur as a result of nonuniformity in the cell size. Thecombustion reactions are exothermic and generate heat. More heat will begenerated in the smaller cells, increasing the temperature of thesubstrate walls and the gas phase which is not desired for turbineoperating conditions.

A uniform cross section other than that of a honeycomb can be obtainedby extrusion or by some other method. An example is a cross section witha circular hole pattern with each circle having the same radius. Thispattern will also allow uniform gas velocity and temperature profile. Avery important advantage of honeycomb profile over any other design isthat the former will produce less back pressure to the incoming gasstream. Obtaining minimum back pressure is very critical in Catcomapplication. Higher back pressure will cause higher compression forcewhich results in fuel penalty for the system. Minimum pressure dropincreases the efficiency of the system. Energy is provided to turbine bythe expansion of the gas phase and is characterized by a PV term. Anincrease in pressure drop results in a decrease in the amount of energyprovided to the turbine.

Finally, in honeycombs, the inner surface of each wall is available forcatalysis.

Ideally, cell sizes should be infinitely small with wall thicknessesinfinitely thin. However, in practice, when walls are made thinner,extrusion becomes more difficult and also strength of the substratediminishes. Therefore, cell size and wall thickness in a honeycomb to beused for Catcom application should be optimized amongst the engineeringconstraints.

An extruded profile with uniform cross section is the preferredstructure as Catcom substrate since extrusion produces a pattern withuniform cross section which is essential in producing and maintaininguniform gas velocity and temperature profile. Honeycomb shape, inparticular, is preferred because this geometry minimizes back pressurewhile it provides high available surface area for catalysis.

Mullite whisker honeycomb substrates for Catcom can be used with variousprecious metal catalysts, including palladium oxide, such as describedin U.S. Pat. No. 4,893,463. A mixture of a refractory inorganic binderand a catalytically effective amount of a binary oxide of the formulaPr₄ PdO₇ is recommended. See U.S. Ser. No. 07/684,631, filed Apr. 12,1991, now U.S. Pat. No. 5,102,639, the teachings of which areincorporated herein by cross-reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be further understood by reference to the drawings inwhich:

FIG. 1 is an X-ray diffraction pattern of a T-0 specimen.

FIG. 2 is a scanning electron micrograph of the surface of a T-0specimen.

FIGS. 3 and 5 are micrographs of T-0 material.

FIG. 4 is a micrograph depicting a joint formed between two pieces ofmullite whisker material.

FIG. 6 is a plot of compression strength of a mullite whisker articlevs. carbon content of reactants.

FIG. 7 is a plot of compression strength vs. temperature for mullitewhisker articles.

FIG. 8 is a plot of relative stiffness vs. temperature for mullitewhisker articles.

FIG. 9 is a micrograph of the structure of a honeycomb produced from thetopaz (T-100) route.

FIG. 10 is a micrograph of the structure of a honeycomb produceddirectly from aluminum fluoride and silica.

FIG. 11 is an SEM photomicrograph at 20,000 magnification showing themicrostructure of microspheres of kaolin clay fired to mullite and thenleached with caustic to remove free silica formed during the reaction.

FIG. 12 is an SEM micrograph of a honeycomb wall of a mullite whiskermaterial with T-50 composition.

To illustrate more completely the invention, the following examples aregiven. These examples are for purposes of illustration only and are notto be construed as limitations of the invention.

EXAMPLE 1

This example illustrates the production of a T-0 honeycomb. Two-hundredand fifty four grams of aluminum fluoride (Aldrich ALF₃.2.8 H₂ O) wasadded to 123 grams of -325 mesh amorphous silica (Thermal American,Montville, N.J.) and ground and mixed in a ball mill for 24 hours. Themix was then placed in a pug mill where 120 ml of 7% Methocel solutionwas slowly added to the batch during mixing. The mixing continued untilan extrudable plastic mass was obtained. The charge was then fed to theextrusion chamber and extruded through a simple honeycomb die, andshapes of approximately 11/2" diameter and 2" length were formed. Thecell density of the pieces were approximately 11 cells/square inch.

A number of such extruded pieces were placed in an oven at 75° C. sothat the Methocel binder gelled. Then the pieces were placed uncoveredin a nitrogen flowing tube furnace and heated at a rate of 10° C./minuntil 350° C. was reached. The pieces were soaked at this temperaturefor one hour and heated at a rate of 10° C./min until 1000° C. wasreached. The samples were then furnace cooled, with room temperaturebeing obtained over the course of several hours. The appearance of thesesamples (referred to herein as 1(a)) was porous, strong enough to handlewithout breaking, cohesive, integral and dark grey in color.

One sample was reheated to 1000° C. at a rate of 8° C./min, followed byheating it to 1300° C. at a rate of 5° C./min. The honeycomb piece washeld at this temperature for 6 hours. Finally, the piece was cooled fromthis temperature to room temperature at a rate of 25° C./min. Thishoneycomb (referred to herein as 1(b)) appeared white, extremely porous,cohesive, integral, strong enough to handle without breaking and verylightweight. The initial and final weights of the honeycomb were 26.13and 8.42 grams, respectively.

Once piece prepared as in example 1(b) was thermal shocked in a gasfired furnace from 1000° C. to 600° C. at a rate of 25° C./sec. Visualexamination showed no evidence of cracking. The same piece wassubsequently shocked from 1100°, 1200°, 1300° and 1400° C. to 600° C. atthe rates of 23.8°, 25°, 25.9°, 26.7° C./sec, respectively. After eachshock, the piece was visually examined and no cracking was observed. Twothree-point flexural beams were cut from this piece. The strength ofeach beam was measured using Instron equipment (Model 4202) at acrosshead speed of 0.0075 in/min. The strength of the porous honeycombspecimens with two cells in width and one cell thickness was measured as111 psi. The apparent density of the honeycomb wall was measured as 0.48gr/cc.

The X-ray diffraction (XRD) pattern of the honeycomb material is shownin FIG. 1. No evidence of glassy phase was observed. The material as isevident from this pattern is very crystalline mullite. Chemical analysisdone using EDX analysis showed the whiskers have a mullite composition(40.0% Al, 11.4% Si and 48.6% O).

The scanning electron micrograph (SEM) of the fracture surface revealsthe microstructure of this material as shown in FIG. 2. The material isbasically an open 3-dimensional whisker structure, with interconnectedwhiskers of sizes ranging from 0.05 to 10 microns or larger in diameter.Individual whiskers appear to be very crystalline with very smoothsurfaces (FIG. 3). The transmission electron micrograph of the whiskersshowed that they are branched and interconnected with very cleanboundaries, exhibiting very little or no glassy phase at the site ofinterconnection (FIG. 4).

The honeycomb pieces were machinable. Four holes were drilled using anelectric drill with a 3/32 drill bit. The holes were 1/4" apart fromeach other. There was no apparent cracking around the holes.

EXAMPLE 2

Discs of green material composed of hydrated aluminum fluoride, silica,Methocel and water were dry pressed into forms 13 millimeter in diameterwith heights varying between 5 to 9 millimeters. The AlF₃ /SiO₂ ratiowas kept at 2/0.968. The discs were fired in flowing nitrogen at 1400°C. Compression strength of the discs was evaluated using a screw drivenInstron equipment (Model 4202) at a crosshead speed of 0.0075 in/min.The average strength was obtained as 310±56 psi. Elevated temperaturetests showed compression strengths at 1200° C., 1300° C. and 1400° C. of306±49, 243±31 and 220-±37 psi, respectively.

EXAMPLE 3

Aluminum fluoride hydrate and silica powders each with top sizes of 45microns were ground separately in anhydrous ethanol in an Eiger millusing zirconia ball media. Handling of aluminum fluoride in a water-freeenvironment at this point is important in order to maintain a flowablepowder. The powders were then ground to less than 10 microns and ovendried and in a molar ratio of 12:13 and mixed in ethanol in the abovedescribed Eiger mill. No ball media was used. The stoichiometric mixturewas then oven dried. Methocel in the form of 0.2% solution was manuallyadded to the mix. Discs were formed, dried in an oven then fired inflowing nitrogen at 1400° C. Room temperature compression strength ofthese discs was 667±41 psi. The described pregrinding of the reactants,which resulted in initial powders finer than 45 microns, thus improvedthe strength of the porous mullite discs.

Backpressure measurements on these discs were performed by flowingnitrogen through them. Back pressures were measured using a differentialpressure gauge placed in parallel to the sample disc. Flow rate of theincoming gas was carefully monitored. Backpressure increased as the flowrate increased. As expected, thicker samples showed higher backpressures. The permeability coefficient of the filter material wascalculated using Darcey's equation. Its value was determined as3.86±1.12 (x10⁻¹³) m².

EXAMPLE 4

Aluminum fluoride hydrate and silica powder were ground and discs wereprepared as described in Example 3. Two discs were placed on top of eachother and fired in flowing air at 1400° C. FIG. 5 shows the interfacebetween the two discs after firing. The joint zone is approximately 20microns. The darker color background is epoxy used in sample preparationfor microscopy. Whiskers grew from each disc and joined the two discs.The joint appears uniform and no glassy phase is apparent at theboundary.

EXAMPLE 5

An alumina fluoride hydrate, silica, Methocel mixture was prepared asdescribed in Example 3, extruded through an 11 cell/square inch die toform a honeycomb and fired in flowing nitrogen 1400° C.

A mullite precursor sol mixture was then prepared using 15 grams of a26% silica and 4% alumina sol mixture (NALCO ISJ-612) and 93.3 grams ofa 10% alumina sol (NALCO-ISJ-614). The sol mixture was stirred for fourdays using a magnetic stirrer. The honeycomb piece was then dipped inthe sol mixture twice, dried at 120° C. and dipped twice again followedby drying at 120° C. The honeycomb was then fired to 1400° C. andweighed upon cooling. A weight gain of 10% was noted. Repeated examplesshowed weight gains of up to 20% or more, demonstrating thatdensification of fired honeycombs could be easily achieved.

EXAMPLE 6

Two sample geometries of mullite whisker material were prepared. Thesewere simple honeycombs (1.5 inch dia., with 11 cells/square inch) anddiscs (13 mm dia.). The apparent densities were varied to observe theeffect of porosity on key parameters such as strength and backpressure.These samples were prepared by adding graphite into the mix, thegraphite burning off at a temperature range of 600°-800° C. resulting inincreased porosity.

1) Strength

Several discs (13 mm dia.) were prepared by dry pressing. Samples wereheat treated in nitrogen at 1300° C. for 12 hours. The averagecompressive strength of the fired discs was 310 psi. The specificstrength (strength/density) was 16610 inches. Samples with greaterporosity, prepared using graphite, had less strength as illustrates in(FIG. 6).

ii) Durability

Strength as a function of testing temperature is shown in FIG. 7. Roomtemperature strength was maintained at 1200° C. A 10% drop was observedat 1300° C. At 1400° C. strength was 220 psi, only 25% less thanobtained at room temperature. The material was still quite usable atthis temperature. FIG. 8 show relative stiffness of the discs as afunction of testing temperature. These values were obtained by measuringthe slope of the elastic portion of the load vs. time curves.

iii) Thermal Shock Resistance

One honeycomb piece, fired at 1300° C., was thermally shocked in a gasfired furnace from 1000° C. to 600° C. at an approximate rate of 25°C./sec. Visual examination showed no evidence of cracking. The samepiece was subsequently shocked from 1100°, 1200°, 1300° and 1400° C. atthe same rate to 600° C. After each shock, the piece was visuallyexamined and not cracking was observed. A second honeycomb was shockedfrom 1300° C. for 5 cycles at a rate of 25° C./sec and ultimatelyfailed. The XRD analysis showed that this piece had cristobalite in itwhich is expected to be detrimental because of high temperature phasetransformations. More severe thermal shock experiments were done withother honeycomb samples. One sample survived water quenching from 100°and 1300° C. at an approximate rate of 280° C./sec. Minor spalling wasobserved along the circumference of the cylinder.

EXAMPLE 7

A commercial calcined kaolin clay (Satintone #5, a fine particle sizecalcined kaolin, marketed by Engelhard Corporation) was mixed withpowdered AlF₃ hydrate (5.52 grams AlF₃, 1.80 grams SiO₂, 2.22 grams clayand 0.48 gram Methocel), pressed into a pellet and fired in nitrogen to1300° C. XRD results showed that the final product was principallymullite, alpha-alumina and a minor amount of cristobalite. Since thepresence of cristobalite may impair thermal shock resistance, it may beadvantageous to compensate for the excess silica in the fired product byadding alumina powder or aluminum fluoride hydrate to the initial powdermix and thereby obtain 100% mullite composition.

EXAMPLE 8

Thirteen grams of a commercial fine particle size hydrous kaolin(Engelhard ASP-172) was added to 465 ml water in a beaker. The beakerwas then heated to 90° C. followed by transferring of the contents to ablender in which the mixture was stirred at low speed. Dry Methocelpowder, 65 gram, was added to prepare a 14% methocel solution and themixture chilled and refrigerated.

Aluminum fluoride hydrate and silica powders, 297.5 and 143.9 grams,respectively, were mixed in a ball mill jar and blended overnight, thedry fluoride/silica mixture pug milled, 145.2 grams of the abovedescribed Methocel solution being added during milling. After 30 minutesof pugging, the mixture was extruded to a honeycomb shape ofapproximately 50 cells per square inch using a piston extruder. Thismixture was softer and far more easily extruded than those extrudedwithout clay addition to the Methocel mixture. Clay addition reduced theamount of water needed to extrude a cohesive, integral piece.

EXAMPLE 9

This example describes a method to produce mullite whisker honeycombusing topaz as the only aluminum source (T-100) in.

A quantity of 348.5 grams of aluminum fluoride trihydrate was mixed in apug mill with 151.5 grams of amorphous anhydrous, fused amorphoussilicon dioxide, from Thermal American, Montville, N.J. (-325 Mesh). Tothis mixture, 21.6 grams of dry Methocel® methyl cellulose was added.The mix was pugged for 15 minutes. A total of 130 mls. deionized waterwas added to the mix. Pugging continued for 30 more minutes. The pastewas then extruded through a multiple die with 1/16" openings using apiston extruder to spaghetti shape. The uncovered extrudates were thenfired in flowing air at 7500° C. for 12 hours. Weight loss upon firingwas 52%. The XRD analysis showed that the sole crystalline phase in thefired product was topaz. A total of 441 grams of topaz was prepared asdescribed above were mixed with 24 grams of silicon dioxide (the same asused above) in a ball mill with liquid medium being ethyl alcohol. Afterbeing ball milled 24 hours, the mixture was filtered and dried. Thedried mixture was mixed with 23 grams of Methocel 20-213 binder andmixed in a pug mill. A total of 140 mls. of water was added to producean extrudable paste. The mix was then extruded through a 50 cpsi 1.5"die to a honeycomb shape using a piston extruder; wall thickness wasabout 0.9 mm. The honeycomb pieces were then fired in flowing air usinga heating schedule of 10° C./min to 350° C.; hold 7 hours at 350° C.;10° C./min to 1000° C.; 8° C./min into 1300° C.; 5° C./min to 1400° C.hold 90 minutes. The furnace was then cooled 25° C./min to 1400° C. toproduce mullite. The final extrudate was stronger than that obtained byextruding aluminum fluoride and silica.

FIGS. 9 and 10 show the microstructures of this topaz produced honeycomband one obtained by firing aluminum fluoride trihydrate with silicaalone, respectively. The denser interconnected whisker network presentin the material produced by the topaz route of this example is apparentfrom a comparison of these figures.

The pieces had large cracks and therefore porosity was not measured;however 35% weight loss was observed in a similar run (see Example 11).Surface area of the product (BET) was 1 m² /gram. Aspect ratio of thewhiskers was greater than 100:1. Whiskers are 100% dense. See whiskerSEM micrographs.

Similar tests were carried out with both nitrogen and air firing to1300°-1500° C. as the peak temperature. Results did not show adifference in the products. For further purposes of comparison, FIG. 11shows the microstructure of a mullite article prepared by firingmicrospheres kaolin to mullite and leaching the free silica. Theparticles on the surface of the microspheres are cigar shaped with anaspect ratio of 3:1 with about 0.1 microdiameter.

EXAMPLE 10

This example shows that topaz induced mullite whisker honeycomb can beproduced by extrusion using a glycol plasticizer as an extrusion aid.

A topaz/silica mixture was prepared as described in Example 9.Polyethylene glycol was added to the dry mixture in amount less than 1%by wt. A total of 5% of dry Methocel binder was added. A total of 110ml. water was added to produce extrudable paste. The mix was thenextruded and fired as described in Example 9. The appearance andproperties of the honeycombs were same as described in Example 9. Theweight lost on firing was 35%.

EXAMPLE 11

This example shows that topaz can be added to aluminum fluoridetrihydrate and silicon mixture, extruded to honeycomb shape and fired toproduce an interconnected mullite whiskers honeycomb. In this example,30% of the mullite whiskers were generated from reaction of topaz andsilica, whereas the rest was formed in situ from reaction of aluminumfluoride trihydrate and silica.

A dry mix was made by blending 249 grams of aluminum fluoridetrihydrate, 120 grams of silicon oxide and 71 grams of topaz prepared asdescribed in Example 9. Methocel binder, 26 grams, was added to the drymix. A total of 107 mls. of water was added and pugged for 40 minutes.The mix was then extruded to a honeycomb shape through a 50 cpsi die viaa piston extruder. The extrudates were then fired at 1400° C. in air.Weight loss of 55% was observed upon firing. The resulting extrudateswere all composed of interconnected mullite whiskers, of which 30% weregenerated by the reaction of topaz and silica. The rest of the whiskerswere formed by the reaction of aluminum fluoride and silica. Thehoneycomb was denser and stronger than the one described in Example 1which is T-0 (no topaz added to initial dry mix). The product of Example11 is designated as T-30 since 30% of the whiskers were generated fromthe barlike topaz added to the initial mix. Examples 9 and 10 describepreparation of T-100 product (i.e. no AlF₃.3H₂ O was used in the initialmix). Only topaz and silica were used to produce honeycombs.

It was found that strength and density were higher when using the highertopaz content in the reactant mix, but porosity was lower.

EXAMPLE 12

A four-component dry powder mix was blended in a pug mill. The mix wascomposed of 249 grams of aluminum fluoride trihydrate, 120 grams offused silica, 71 grams of topaz prepared as described in Example 9, and22 grams of dry Methocel powder. Polyethylene glycol (Dow ChemicalPolyglycol E-400) was added, in amount of 0.5 wt. % of dry materials, tothe mixture. A water volume of 91 mls. was added to the mix and puggedfor 45 minutes. A second batch was made following the same procedure todouble the size of the paste. Both batches were fed to a twin screwextruder and honeycomb pieces with 64 cpsi and 2"×2" were extruded. Someof the extrudates were air dried while others were dried in aconventional kitchen type microwave oven at medium heat. Microwave driedpieces appeared to be smoother on the outside surfaces with fewercracks. Some pieces were heated in the microwave in the presence ofwater vapor which was generated by water present in a one liter beakerlocated inside the microwave cavity. These pieces had the least numberof flaws on the outside. The pieces were then fired at 1400° C. to havethe same composition as described in Example 11.

EXAMPLE 13

This example describes the preparation of a T-50 honeycomb using atwin-screw extruder where mixing and extruding are both done using thesame piece of equipment. Aluminum fluoride was slightly more than thestoichiometric amount required to make T-50 product to compensate forthe volatile fluorine loss at elevated temperatures before the onset ofthe reaction of aluminum fluoride with silica.

A total of 24.4 lbs. of aluminum fluoride trihydrate was blended with15.7 lbs. of topaz prepared as in Example 9 in a sigma mixer. This mixwas fed dry via a loss-in-weight type feeder to a co-rotating twin screwextruder at a rate of 73.8 lbs./hr. A mixture of silica (same asExample 1) and dry Methocel, at a ratio of 100:15, was fed at a rate of26.2 lbs./hr. simultaneously. A water/glycol mixture (2.7% glycol) wasalso independently fed to the extruder. The powders and liquid weremixed and conveyed through the chilled barrels of the extruder andthrough a 2"×2" 64 cpsi honeycomb die. The extrudates were then slicedto various lengths and dried either in air or in microwave oven in thepresence of water vapor as described in Example 9. The extrudates didnot show cracking on the outside surfaces and fired to forminterconnected mullite whiskers, of which 50% was generated by thereaction of topaz and silica.

EXAMPLE 14

This example shows that colloidal silica can be used to replaceamorphous fused silica powder to produce shapes which produces strongerproduct.

A quantity of 9.6 grams of topaz prepared as described in Example 9 and0.4 grams of dry Methocel powder was mixed with 1.92 grams of hydrouscolloidal silica (NALCO-1050) containing 50% silica by weight, thebalance being water. Water was added gradually so that the paste had 21%water. The mix was then pressed to 1.25" diameter discs at a pressure of5000 psi at 80° C. The discs were then fired and cut to make bend barswith approximately 6.8×3.3 mms. cross section. The bars were tested forstrength by three point bending tests and their strength varied between7000-9000 psi. Strength of the material made using colloidal silica washigher by 2000-3000 psi than that made by amorphous powder silica.

EXAMPLE 15

A total of 450 grams of aluminum fluoride trihydrate, silica/Methocelmixture and -100 mesh topaz prepared as in Example 9, was blended in thesame ratio used in Example 13 in a sigma blade pug mill. To this mixturewater/glycol mixture was added as a 2.7% glycol solution and the mixturewas pugged until a paste suitable for extrusion was obtained. The pastewas then pressed through a 50 cpsi die using a ram press to obtainhoneycomb shaped profiles of 1.5" diameter. The pieces were then driedand fired in air at a temperature of 1400° C. for full conversion ofreactants to mullite. One piece was then coated with a standardcommercial precious metal containing autocatalyst washcoat based onalumina. The piece was then dried and calcined to observe adhesion ofcatalyst washcoat to the substrate. The washcoat adhered to thesubstrate uniformly after calcining. The piece was then tested forcatalytic activity for C₇ H₁₆ oxidation, SO₂ to SO₃, CO to CO₂, and NOto NO₂ using a diagnostic reactor. The results of the conversion arelisted below:

    ______________________________________                                        Species        Conversion Rate                                                ______________________________________                                        C.sub.7 H.sub.16 oxidation                                                                   37%                                                            SO.sub.2 to SO.sub.3                                                                         23%                                                            CO to CO.sub.2 62%                                                            NO to NO.sub.2  3%                                                            ______________________________________                                    

These results demonstrate that mullite whisker honeycombs can be coatedwith commercial precious metal based washcoats and the catalystdeposited on the substrate is active in oxidizing hydrocarbons, O, NOand SO₂. Also, the catalyst deposited on mullite whisker material isactive and oxidizes hydrocarbons, CO, NO and SO₂.

EXAMPLE 16

This example describes a method to join two pieces of fired honeycombsto form larger pieces using topaz to form a strong, dense joint.

55.2 grams of topaz, produced in a whisker furnace in spaghetti form, asdescribed in Example 9, was mixed with 3.0 grams of fused silica (ThermoAmerican, -325 Mesh) in a 200 cc capacity ball mill filled halfway withalumina balls. Water was added to cover the balls and the mixture wasmilled for 48 hours to a very fine powder suspension. The slurry wasthen recovered and placed in a closed jar. The jar was not disturbed forseven days. It was observed that the contents in the jar were gelled.

Two mullite honeycomb pieces with 100 cpsi and T-50 composition weresliced parallel to the honeycomb wall direction. The topaz/silica gelwas then spread on both faces of the honeycomb walls and two pieces werepressed under 1 kg load overnight. The joined dry piece was then firedin the whisker furnace in air to 1400° C. for 12 hours. The productappeared to have a solid joint upon firing.

Various changes and modifications can be made in the process andprocesses of this present invention without departing from the scope andspirit thereof. The various embodiments disclosed herein are for thepurpose of further illustrating the invention but are not intended tolimit it.

We claim:
 1. A process for forming a honeycomb-shaped article comprisinginterconnected mullite whiskers comprising the steps of:a. preparing amixture of a fugitive binder, hydrated aluminum fluoride and silicondioxide, the proportions of hydrated aluminum fluoride and silicondioxide being approximately stoichiometric to form topaz; b. formingsaid mixture from Step (a) into a coherent green body; c. firing saidbody from Step (b) until said body is converted substantially completelyinto interconnected bar-like topaz crystals; d. recovering said bar-liketopaz crystals and mixing them with a material capable of formingmullite by reaction with topaz, said material being selected from thegroup consisting of silicon dioxide, a mixture of silicon dioxide andhydrated aluminum fluoride, and a mixture of silicon dioxide, hydratedaluminum fluoride and alumina, together with a fugitive binder; e.forming the mixture from Step (d) into a honeycomb; and f. firing saidhoneycomb at elevated temperature while removing volatiles as they areformed until substantially complete conversion of said honeycomb tomullite whiskers occurs, the firing in Steps (c) and (f) being carriedout in a furnace provided with an external peristaltic pumping systemwhich continuously sucks gases formed during the reaction from insidethe furnace to outside the furnace, and gases sucked from the furnaceare charged to a scrubber unit.